Homeland Security Secretary Janet Napolitano said Wednesday that BP — and only BP — can stop the gush of oil into the Gulf of Mexico caused by the April explosion of an offshore drilling rig.

"They have got to plug that well," she told reporters at a Kansas City news conference. "They have the technology, they have the personnel and experience. The government doesn't have that capacity."

The company tried again to plug the leak Wednesday by covering it with heavy mud, a process known as a "top kill." A company official said it could be several days before results will be known. The process has never been tried on an oil well nearly a mile under water.

A chorus of critics has urged the Obama administration to take a more active role in trying to stop the leak, now considered one of the largest environmental disasters in American history.

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"I have no idea of why their attitude is so hands-off here," said Democratic operative James Carville, speaking on ABC. "It's just unbelievable."

Napolitano, in the Kansas City area to discuss homeland security preparedness, said the government isn't capable of stopping the oil, but has taken an active role in mitigating the damage.

"This is the largest federal response ever to an environmental disaster of this sort," she said. "We have right now over 22,000 federal personnel on that aspect of it."

The former Arizona governor said the government will review potential changes in law and regulations to be prepared for any future oil well ruptures. A government report on the spill is possible today.

President Barack Obama is expected to tour the area Friday.

At the short news conference, Napolitano supported the administration's decision to send additional troops to the southern border, where the political battle over illegal immigration shows no signs of abating. She also urged Congress to take up comprehensive immigration reform.

Napolitano said she hasn't read the entire Arizona bill, which supporters maintain allows law enforcement officers to more easily identify people in the country illegally.